1. Field of the Invention
The present invention generally relates to printers, and more particularly, to a printer capable of printing at regular time intervals.
2. Description of the Background Art
A facsimile apparatus is generally provided with a linear thermal head which thermosensitively records image data transmitted from another facsimile apparatus on thermosensible paper or the like. Such a linear thermal head comprises a plurality of heating elements which are arranged in a line and selectively heated based on printing data.
A facsimile apparatus in general transmits image data according to a telecommunication system standardized by the CCITT (International Telegraph and Telephone Consultative Community). For example, in a telecommunication system standardized by the CCITT as "G III", there are provided a telecommunication mode called "standard mode" for standard image data and another mode called "fine mode" which enables transmission of high definition image data.
FIG. 4 is an illustration for explaining printing operation of a facsimile apparatus in the fine mode. As shown in FIG. 4, a printing head of the facsimile apparatus has a plurality of heating elements Hj (j=1, 2, . . . , n) arranged thereon, above which recording paper 22 is fed in the direction of arrow 21. The heating elements Hj have a width of L1 in the feeding direction 21 of the recording paper 22. When a one-line printing is completed by the heating elements Hj, the recording paper 22 is conveyed only by the width L1. In the fine mode, therefore, lines l1 and l2 each having the width L1 are printed based on image data corresponding to the lines l1 and l2.
FIG. 5 is an illustration for explaining printing operation of a facsimile apparatus in the standard mode. In the standard mode, one line l3 has a width of L2 twice the width L1. Accordingly, the heating elements Hj of the width L1 can not accomplish printing of the one line l3 at a time, so that two printing operations are performed based on image data for the one line. Therefore, a printing operation in an area indicated by reference character l3a and another printing operation in an area indicated by reference character l3b are performed separately.
FIG. 6 is a timing chart for explaining printing operation of a facsimile apparatus in the standard mode, which is taken as a background art of the present invention. In the standard mode, after received image data of one line are decoded, printing is executed twice based on those one-line image data. For example, as shown in FIG. 6 (1), when the n-th line image data are decoded during a period W1, upon start of the decoding of this n-th line image data, the first printing of the (n-1)-th line is started.
As shown in FIG. 6 (2), the first printing of the (n-1)-th line is executed during a period W2, and upon completion of this first printing, the second printing of the (n-1)-th line is executed during a period W3. After a period of W4 has passed since the completion of the second printing, decoding of the (n+1)-th line image data is started, and at the same time, the first printing of the n-th line image data that have been decoded during the period W1 is also started, followed by the second printing.
According to the printing system employed in the facsimile apparatus above in the standard mode, when the first printing of the (n-1)-th line is completed, recording paper is conveyed only by a width L1. At this time, a plurality of control signals are selectively switched in their level and applied to a pulse motor for conveying the recording paper so as to rotate the motor only by a certain angle. In practice, however, it takes several milliseconds for the pulse motor to be rotated only by a certain angle on reception of such signals, and stop. Accordingly, the recording paper has not been conveyed enough when the second printing of the (n-1)-th line is started. In addition, during the period W4 between the completion of the second printing for the (n-1)-th line and the beginning of the subsequent printing for the n-th line, the printing paper is only conveyed but no printing operation is performed thereon. Therefore, the first printing is executed after the recording paper has been conveyed enough to accept the subsequent printing. Furthermore, since the period W4 is a relatively long time, when compared with a time interval between the time t.sub.2 when the recording paper is conveyed for the second printing of the (n-1)-th line and the time t.sub.3 when the same is conveyed for the first printing of the n-th line, another time interval between the time t.sub.3 and the time t.sub.4 when the recording paper is conveyed for the second printing of the n-th line is longer. This means that the recording paper is conveyed at timings where a short and a long idle periods appear alternately. The facsimile apparatus that has been taken as a background art of the present invention has, therefore, a problem that irregularities in amount and timings of feeding the recording paper in the printing operation deteriorate printing quality.